1. Field of the Invention
This invention relates to the field of corrosion assessment in a metal by monitoring electrical activity between the metal and an electrode, both being subjected to an electrolyte. More particularly, the invention concerns monitoring localized corrosion of circular structural penetrations of vessels, for example the structures penetrating the vessel of a nuclear reactor for coupling control and sensing devices through a vessel wall.
2. Prior Art
There are various reasons for providing structures penetrating vessels, such as a conduit attached to an opening in the vessel at one or more welds. The conduit may be arranged, for example, to provide a flow path, to pass a mechanical device, or to couple electrical conductors through a vessel wall. Where the vessel contains an electrolyte and is subject to thermal and/or mechanical stress, the penetration structure is subject to corrosion. Where an asymmetric weld or similar structure affixes a penetration to a pressurized vessel containing an electrolyte, corrosion-stress cracking can be a problem, particularly in the area of the attachment of the penetration structure to the vessel.
A pressurized water nuclear reactor vessel is an example. A pressurized water nuclear reactor comprises a pressure vessel containing nuclear fuel and a conduit system whereby a coolant such as water or water containing cobalt is fed into and out of the vessel. The fuel comprises a plurality of long vertical rods that are closely spaced and interspersed with control rods that are movable into the spaces between the fuel rods to damp a selected proportion of nuclear flux and thereby control the rate of nuclear decay of the fuel as well as the amount of heat generated. The coolant is fed to the bottom of the vessel, flows upwardly over the fuel rods and exits at a level above the fuel rods, being heated in the process. The coolant is heated to a high temperature (e.g., 600.degree. F.) and develops substantial pressure (e.g., 2,200 psi).
Instrumentation and control devices are mounted in and on the reactor vessel to ensure proper operation. Although it is possible to provide penetrations of the reactor vessel wall at various places to accommodate the mechanical and electrical couplings needed for these devices, it is preferable to place the penetrations at the top of the vessel and thereby provide higher integrity at the bottom, for improved safety characteristics. Typically, the reactor vessel comprises a generally cylindrical hollow body of relatively thick steel (e.g., about 13 cm or 5 inches) with a lid or reactor vessel head attached at the top to define a sealed pressure vessel. Penetrations for mechanical and electrical couplings across the pressure boundary are provided in the form of fittings that extend through the head or lid of the reactor vessel.
Penetrations of the vessel head are provided, for example, to accommodate movable control rod guide devices, and to pass conductors that couple electrically to sensors located inside the vessel, such as temperature and nuclear flux level sensors disposed in thimble tubes interspersed among the fuel rods. The tubes or similar structures penetrating the head may terminate flush with the inside wall of the head or may protrude into the internal volume. They may also traverse the plane of the head perpendicularly or at an angle. Typically, the penetration tubes are aligned vertically, parallel to one another and in alignment with the fuel rods. The control rods, for example, can thereby be engaged by actuators that are movable upwardly or downwardly in the penetration tubes, to accomplish corresponding displacement of the control rods relative to the fuel.
Whether movable or static, the structures passing through the vessel head have associated penetration tubes that are arranged to withstand the pressure developed by the coolant and to maintain a pressure boundary. The penetration tubes extend through the vessel head and include pressure fittings adapted to pass the control rod mechanisms or electrical signal couplings, respectively. The collection of penetrations and couplings are known as the reactor vessel head adaptor.
All the tubing and conduits of the reactor which carry the coolant are subject to corrosion over time. There are a number of reasons for such corrosion, including chemical reaction with the coolant (which is an electrolyte), the effect of nuclear radiation, mechanical stresses due to temperature and pressure variations, etc. Penetrations of the reactor vessel head are subject to stress-corrosion cracking, leading to potential leakage.
The vessel head is dome shaped, as appropriate for withstanding pressure. Whereas the penetrations typically are vertical and the head is a dome, those penetrations which are radially spaced from the center of the dome pass through the dome at an angle relative to a tangent to the dome surface. Welds which attach the penetration tube to the vessel head are therefore not placed at the same axial level on the penetration tube, and/or are characterized by different sized weldments on opposite sides of the tube. This is especially pronounced at the penetrations located at the outer radius of the vessel head. Thermal stresses are created by the welds for this reason. The thermal stresses further subject the penetration tubes to stress-corrosion cracking, and potential leakage of coolant as a result of through-wall stress corrosion cracking of the tubes, particularly in the area adjacent axially spaced or differently sized welds.
It would be beneficial to monitor cracking at vulnerable locations such as the vessel head penetrations in an on-line and automated manner, to determine when actual crack propagation is taking place and to assess the effectiveness of corrective measures taken to arrest corrosion of the reactor vessel. An on-line crack monitoring system could also provide information to evaluate the relative severity of corrosion at different locations in the reactor head to help determine and correct the root causes.
Electrochemical corrosion measurements have been taken to generally monitor the level of corrosion of metals that are exposed to an electrolyte. In high temperature environments such as boilers, corrosion may be encountered due to exposure to flue gases or to an aqueous coolant. U.S. Pat. No. 4,575,678--Hladky discloses a general method for analyzing deterioration of metal structures carrying electrolytes, for example, a pipe or conduit, a storage tank, process vessel, heat exchanger, pump or valve. An electrochemical probe intended for ongoing collection of corrosion data, that protrudes from a vessel wall into the electrolyte, is disclosed for example, in international application PCT/GB87/00500--Cox et al. A probe that is structured to form a section of conduit through which the electrolyte passes is disclosed in U.S Pat. No. 4,426,618--Ronchetti et al. In each case, the probe comprises a plurality of corrosion sensing electrodes that are exposed to the electrolyte. The electrical potentials of the electrodes and the current passing between the electrodes is sensed and related to the extent of chemical corrosion of the electrodes. Corrosion of the electrodes is comparable to corrosion of the vessel, conduit or other structure that holds the electrolyte. Therefore, by sensing the level of corrosion of the electrodes and integrating the results, the probe can be used to estimate the instantaneous rate of corrosion of the structure as a whole. Such information can be incorporated as a part of a maintenance program as disclosed in U.S. Pat. No. 4,935,195--Palusamy et al.
Specific sensing and monitoring for electrochemical resistance, galvanic current between electrodes, electrochemical potential noise and electrochemical current noise are disclosed for measuring the deterioration of a combustion vessel in "On-Line Materials Surveillance for Improved Reliability in Power Generation Systems," Paper No. 254, NACE Annual Conference and Corrosion Show, March 1991. Electrode structures which are useful for such monitoring are disclosed, for example, in U.S. Pat. Nos. 3,504,323--Meany, Jr.; 3,491,012--Winslow, Jr.; and 2,834,858--Schaschl. These teachings and these patents, and the foregoing patents to Palusamy and Hladky, are hereby incorporated in their entireties.
The present invention is intended to apply the art of electrochemical monitoring to the specific problems of penetration structures traversing the walls of vessels, and is particularly applicable to monitoring localized corrosion of penetrations traversing a wall of a nuclear reactor vessel. It has been discovered according to the invention that by instrumenting a subset of a plurality of penetrations of a vessel or similar electrolyte-holding structure, in particular the control and instrumentation penetrations of a nuclear reactor vessel head, one can assess the status of the penetrations generally, and thereby obtain information on this critical area of the reactor. Furthermore, the corrosion characteristics of particular areas of the penetration tubes can be selectively monitored, for distinguishing corrosion occurring at different locations in the penetration tubes.
The invention is useful for assessing present corrosion level, status and integrity of the vessel penetrations, and for providing information whereby the useful life of penetration structures can be decremented for planning and executing required maintenance steps.